feat: Add support for HTML emails in outgoing messages (#12662)

This PR adds sending custom HTML content in outgoing email messages
through Chatwoot's Email channels, while maintaining backward
compatibility with existing markdown rendering.

###  API Usage

**Endpoint:** `POST
/api/v1/accounts/{account_id}/conversations/{conversation_id}/messages`

```json
  {
   "content": "Fallback text content",
   "email_html_content": "<div><h1>Welcome!</h1><p>This is <strong>custom HTML</strong></p></div>"
  }
```

---------

Co-authored-by: Muhsin <muhsinkeramam@gmail.com>

app/builders/messages/message_builder.rb

@@ -178,7 +178,13 @@ class Messages::MessageBuilder
     email_attributes = ensure_indifferent_access(@message.content_attributes[:email] || {})
     normalized_content = normalize_email_body(@message.content)
 
-    email_attributes[:html_content] = build_html_content(normalized_content)
+    # Use custom HTML content if provided, otherwise generate from message content
+    email_attributes[:html_content] = if custom_email_content_provided?
+                                        build_custom_html_content
+                                      else
+                                        build_html_content(normalized_content)
+                                      end
+
     email_attributes[:text_content] = build_text_content(normalized_content)
     email_attributes
   end
@@ -213,4 +219,17 @@ class Messages::MessageBuilder
 
     ChatwootMarkdownRenderer.new(content).render_message.to_s
   end
+
+  def custom_email_content_provided?
+    @params[:email_html_content].present?
+  end
+
+  def build_custom_html_content
+    html_content = ensure_indifferent_access(@message.content_attributes.dig(:email, :html_content) || {})
+
+    html_content[:full] = @params[:email_html_content]
+    html_content[:reply] = @params[:email_html_content]
+
+    html_content
+  end
 end